This application is related to slot-array antennas, in particular, to wide-bandwidth long-slot antenna arrays. Slot-array antennas have apertures theoretically capable of maintaining a constant driving impedance of 377 ohms (Ω) over a wide-bandwidth, for example, over a bandwidth greater than Fmax−0.01*Fmax (i.e., 100:1). However, conventional long-slot antenna arrays are limited by their backplanes and antenna feeds. Conventional antenna arrays are not suitable for many wide-bandwidth applications because they have narrow-bandwidth and/or are physically too thick. Patch antennas generally have a lower profile, but lack sufficient bandwidth necessary for many applications.
In contrast, tapered-slot antenna arrays, analogous to horn antennas, have wide-bandwidth but require considerable depth. In particular, tapered-slot antenna arrays have tapers which may extend behind the radiating elements over a distance of a wavelength or more. It is necessary to use long taper lengths to achieve wide-bandwidth because the taper provides a transition which matches the impedance of the antenna array's transceiver electronic modules and feed lines to the impedance of the environment. The longer the transition between the impedance of the transceiver and the environment, the greater the bandwidth the antenna array can achieve. Thus, conventional taper elements obtain wide-bandwidth at the expense of long taper lengths and increased antenna thickness and overall size.
High performance surveillance and other critical missions benefit from ultra wide-bandwidth (UWB) capabilities in the Ultra High Frequency (UHF) spectrum and below. Furthermore, they require high resolution, diversity, and/or multi-radio-frequency (RF) functionality on platforms where antenna volume and/or footprint is limited. However, since UHF radiation has wavelengths on the order of 1 meter, conventional wide-bandwidth tapered slot antennas are large, costly, and impractical.
Other conventional UWB long-slot antenna arrays provide impedance transformers in discrete circuits behind the backplane. Similarly, the thickness of these antenna arrays is increased and may be greater than desired. Furthermore, conventional apertures use radiating elements that required balanced feed lines, such as twin lead cable, which has two parallel conductors formed within an insulating material, similar to a ribbon-cable. When a balanced antenna, such as a dipole, is fed with an unbalanced feed line (e.g., coaxial cable) undesirable common mode currents may form between the inner and outer conductors. As a result, both the unbalanced line and the antenna may radiate, which may reduce efficiency, distort the radiation pattern of the antenna array, and/or induce interference in other electronic equipment.
In order to convert an unbalanced feed line to a balanced feed line, conventional antenna arrays have used a balun. Conventional baluns, however, are expensive, inefficient, and have limited bandwidth and power capability. Additionally, although some conventional UWB long-slot antenna arrays do not require a balun, it may be necessary to provide the antenna array with a thick and heavy dielectric radome for impedance matching.
Accordingly, conventional antenna arrays are insufficient and unsuitable for certain applications since they require balanced feed lines or radomes, do not have a low profile or wide-bandwidth, and/or are not capable of operating over low frequencies. Therefore, antenna arrays having greater performance and smaller profiles, particularly less thickness in the direction of propagation are desired.